Advanced Structural Designs

ACN 097 789 87 92 Vasey Cres CAMPBELL ACT 2612

Telephone 61612171 Facsimile 61612170

Email mal@structuraldesigns.com.au

How far apart can I space my joists if I am using tongue and groove timber flooring?

Firstly it is worth noting that strip flooring comes in 19, 25 and 30 mm thicknesses and five separate seasoned strength groups so the solutions are numerous. Let’s start with residential applications as we generally deal with commercial applications on a case by case basis because the uniformly distributed and point loads vary with each application. It may also be worth pointing out that given the cost of strip flooring most people are choosing it for its hardness and appearance rather than it’s spanning capacity. Spotted gum and ironbark are the hardest, Cypress is moderately hard and hoop pine is about the softest.

Residential Construction

The answer most people give to this question is 450mm as this is an industry standard and can cover a few abnormal load patterns in most 19mm thick boards (cypress is generally 20mm). There are however occasions when the floor joists are exposed as an architectural feature that you might like to spread them further apart. If you are looking to insulate the underfloor space you may also find 600 mm centres suits the standard batt widths.

Loading

For the exercise let us assume that the area is residential, abnormal loads such as spa baths and waterbeds are not being considered and that the area will not be used for storage. Lets us also assume that the joints will be butt joined over joists. This gives a stronger result than the end matched flooring profiles but requires a minimum of 45 to 50 wide floor joists to reduce the potential for end splitting. In these circumstances you could expect select grade Jarrah to require a joist spacing of 680mm yet some pine species would require joists at 510mm. The answer is further reduced if the joists are cold formed sections rather than timber.

The trap some designers fall into when looking at this question is to look at tables of uniform distributed load capacity for say a span/360 deflection. This is misleading on two counts, firstly it does not take into account point load requirements and secondly it does not take into account dynamic response criteria. Floors selected on this basis can result in local failures or leave the occupants with a perception that their floor is too “springy”. If you need your floor to be designed for a commercial application or need to know any information regarding species, moisture content, storage, expansion joints, subfloor ventilation or proximity to the ground ring Advanced Structural Designs on 61612171. We can have a structural engineer on site in the Canberra ACT region the next day.